Post-combustion devices for internal combustion engines

ABSTRACT

The device comprises a post-combustion chamber bounded by a casing and connected to the engine exhaust pipe, a blower driven by the engine for sending additional air into the post-combustion chamber and a heat-exchanger for pre-heating the additional air by the exhaust gases from the post-combustion chamber. The poscombustion chamber and the heat-exchanger are constituted by three co-axial cylinders, the assembly being arranged so that the exhaust gases are introduced into the axis and at the center of the post-combustion chamber, the outer cylinder and the intermediate cylinder defining a peripheral zone in which the additional air circulates. The intermediate cylinder and the inner cylinder define an intermediate zone in which the exhaust gases from the combustion chamber circulate in countercurrent to the additional air. The device is useful for two-stroke, fourstroke and rotary piston engines.

United States Patent 151 3,704,592 Panhard [451 Dec. 5, 1972 s41 POST-COMBUSTION DEVICES FOR 3,392,523 7/1968 Hyde ..60/30 R INTERNAL COMBUSTION ENGINES 3,302,394 2/1967 Pahnke ..60/30 R [72] Inventor: Jun Palm-rd, Paris, France 3,601,982 8/1972 McCrocklm ..60/30 R [73] Assigneei Societe de Construction Mecaniques Primary Examiner-Douglas Hart Panhard & Levassor, Paris, France Att0meyWaters, Roditi, Schwartz & Nissen [22] Filed: July 16, 1970 [57] ABSTRACT [211 App! 55408 The device comprises a post-combustion chamber bounded by a casing and connected to the engine ex- [30] Foreign Application Priority Data haust pipe, a blower driven by the engine for sending additional air into the post-combustion chamber and a July 17, 1969 France ..6924407 heat exchanger for reheating the additional air y the exhaust gases from the post-combustion chamber. [52] US. Cl ..60/290, 23/277 C, 6242229948, The pos combustion chamber and the heabexchanger tituted by three co-axial cylinders, the as- [51 Int. Cl. ..F02b 75/10, FOln 3/10 are sembly being arranged so that the exhaust gases are [58] Field of Search "60/30 2 introduced into the axis and at the center of the postcombustion chamber, the outer cylinder and the intermediate cylinder defining a peripheral zone in which [56] References Clted the additional air circulates. The intermediate cylinder UNITED STATES PATENTS and the inner cylinder define an intermediate zone in which the exhaust gases from the combustion chamber 1,605,484 11/1926 circulate in countercurrent to the additional air. The 1348990 3/1932 device is useful for two-stroke, four-stroke and rotary 1,877,523 9/1932 piston engines 3,091,081 5/1963 Alper ..60/267 3,285,709 1 1/ 1966 Eannarino ..60/29 A 10 Claims, 4 Drawing Figures PATENTED UEC 5 I972 SHEU 1 OF 3 PATENTEDDEI: 5M

SHEET 2 I 3 The invention relates to post-combustion devices for burning combustible gases contained in the exhaust gas of an internal combustion engine of the type which comprise:

a post-combustion chamber limited by a casing and linked to the exhaust piping of the engine,

means for introducing additional air into the post combustion chamber,

a heat exchanger for pre-heating the additional air by exhaust gas from the post-combustion. chamber, the heatexchanger surrounding the said chamberfor postcombustion and being such that the circulation of the additional air is done in a peripheral zone in countercurrent to the exhaust gases which circulate in an intermediary zone between the said periphery zone and the casing of the combustion chamber.

The invention concerns more particularly, because it is in this case that its application seems likely to have the most interest, but not exclusively, amongst these devices for post-combustion,,to those for internal combustion engines in which, in the course of the cycle, there is a simultaneous opening of admission and of exhaust, especially two-stroke engines, four-stroke Diesel engines and rotary piston engines.

It has already been proposed to eliminate or to reduce the undesirable constituents of the exhaust, gas of internal combustion engines by burning these constituents, especially carbon monoxide, with the aid of additional air mixed with these exhaust gases, upstream of the post-combustion chamber. However, the efficiency of devices of the type described above, known to this day, is not entirely satisfactory and very often a spark plug is necessary to ignite the mixture of combustible gas and of additional air in the post-combustion chamber.

It is an object of the invention is to make the postcombustion devices of the type described above sufficiently efficient for the spark plug in the post-combustion chamber to be unnecessary.

Furthermore, the proportion of undesirablev constituents such as carbon monoxide or unburnt hydrocarbons, contained in the exhaust gas from the engine,is high enough at slow speeds of rotation of the engine but relatively'weak at higher speeds of rotation to which correspond the higher levels of powerof the engine. For these powerful levels of operation; the temperature of the exhaust gas before post combustion is already high; in these conditions, after post combustion of a part of the undesirable constituents, even if the latter are not great in quantity, the temperature of the exhaust gas after post-combustion .becomes too high and risks putting'the exhaust manifold into a dangerous state.

It is another object of the invention to remedy this drawback.

According to the invention, a post-combustion device for burning combustible gases contained in the exhaust gas of an internal combustion engine of the type described above, is characterized bythe fact that, on one hand, the means for introducing the additional air comprise a blower adapted to be driven by the englue and to deliver a flow "of air in excess and on the other hand, the assembly of the post combustion chamber and of the heat exchanger is constituted by three coaxial cylinders, that is an interior cylinder forming the casing of the post-combustion chamber and linked to the exhaust piping of the engine in such a way that the exhaust gases of the latter are introduced into the center and into the axis of this cylinder, an intermediary cylinder forming, with the interior cylinder, an annular space which constitutes the abovesaid intermediary zone and communicates, at one end situated on the side of the exhaust piping of the engine, with the post-combustion chamber by openings provided in the internal cylinder and, at its other end, with a piping for the escape of the burnt gases from the post-combustion chamber, an external cylinder defining with the intermediary cylinder the abovesaid peripheral zone, which communicates, on one hand, with an intake piping for additional air and, on the other hand, with the exhaust pipe of the engine by holes provided in the latter and situated axially between two covers substantially flat and perpendicular to the common axis of the cylinders, the said covers being traversed by the exhaust pipe of the engine and being able to shut, at one end, on one hand the intermediary cylinder and the interior cylinder and, on the other hand, the external cylinder, the axial distance between the covers being just sufficient in order that the holes might issue between the said covers, the said peripheral zone comprising a partition rolled in a helix adapted to increase the distance traveled by the additional air in this zone, the assembly being arranged in such a way that the abovesaid intermediary zone separates the peripheral zone from the combustion chamber along the whole of the axial length of this latter.

The post-combustion device comprises means sensitive to the speedand/or to the load of the engine to prevent the air delivered by the blower which has cooled the exhaust gas of the post-combustion chamber and this chamber, from being introduced into the said post combustion chamber when the speed of rotation of the engine and/or the load have values higher than the given limit to which correspond a given limit of power of the engine, in such a way that the post-combustion ceases when the motor delivers a power higher than the limit of power chosen.

The invention consists, apart from the features described hereabove, in certain other features which are used preferably at the same time and about which more will be said explicitly in the detailed description which follows, given with reference to the annexed drawings.

FIG. 1 of these drawings represents schematically an internal combustion engine which is equipped with a post combustion device constructed according to the invention.

FIG. 2 shows longitudinally in detail the post-combustion chamber and the exchanger of the device in FIG. 1.

FIG. 3 represents, in section, a valve belonging to the above-mentioned means sensitive to the speed and to the load of the engine.

FIG. 4, finally, represents, also in section, a variation of the valve of FIG. 3.

As shown on FIG. 1, a internal combustion motor 1 drives a blower 2, or a compressor, which drives additional air into a post-combustion chamber 3.

. 3 The. post-combustion Chamber is lodged in an external casing 4 linked to the blower by a'piping 5. The air circulates in this piping and in the casing 4 according to chamber 3.

An intermediary casing 11 is disposed between the external casing 4 and the walls of the chamber 3, and a partition 1 2.is rolled in a helix between the casings 4 and 11.

As one can see in FIG. 1, the air which leaves the exchanger 6 is obliged, by the partition 12 to follow a sinuous journey between the casing 4 and the walls. of the post-combustion chamber 3. It can then penetrate into the piping 8, linking the exhaust of the engine 1 to the chamber 3, due to the. orifices9 pierced in the said piping 8.

A piping 10 constitutes the exit from the post combustion chamber 3.

The partition 12 obliges the air coming from the blower 2 to turn several times around the cylindrical surface of the wall 11 before penetrating into the postcombustion chamber 3. The wall 11, which is internally, in contact'with the exhaust gases of the postcombustion chamber, transmits a .part of the heat of these gases to the air flowing from the blower 2. The heat-exchanger 6, in this case, is thereforeconstituted by-the assembly of the two casings 4 and 11 and of the partition 12. I

The; functioning of a engine equipped with such a device results clearly from the foregoing description. The exhaust gases from the enginearrive through the pipe 8 and mix, at the position of the orifices 9, with the air flowing from the blower 2. A part of the undesirable constituents burns in the post-combustion chamber 3. The exhaust gasesfrom the. post-combustion chamber heat the air flowing from the blower, then are evacuated by the piping l0. I

' However, if one lets the post-combustion function for speeds of engine rotation relatively high and for maximal engine power, there is a risk of provokinga deterioration of the post-combustion device because of the temperatures, which are too high, of the exhaust gases. Now, the post-combustion becomes unnecessary for, on one hand, at the maximal power of the engine, the amount of undesirable constituents in the exhaust gases is lower than at low power and on the other hand, the maximal power is not in general, attained until outside crowded areas, such that an elimination of undesirable constituents which is not as good, becomes less troublesome.

The. post-combustion device comprises means 13 sensitive to the speed and/or to the load of the engine to prevent air flowing from the blower 2, and which has cooled the exhaust gases and the post-combustion chamber 3, from being introduced into the said postcombustion chamber when the speed of rotation of the engine and/or the load have values higher than a given limit to which corresponds a chosen power limit which has the effect of stopping post-combustion for engine powers higher than the chosen power. limit.

According to one embodiment represented-in FIG. '1, the means 13 are linked tothe position of the accelerator lever of the vehicle, that is to say in practice, linked to the position of the throttle valve 14 situated in the intake pipe 15 of the engine.

In another embodiment (not represented) the means 13 are linked to the flow pressure of the blower 2, which flow pressure depends on the speed of engine rotation of the engine 1.

The means 13 comprise a throttle member constituted by a valve 16 able to open or shut a pipe 17 of which one end can be linked to the silencer (not shown) and of which the other issues into the casing'4.

The air, flowing from the blower 2 can be thus' directed towards the atmosphere by the exhaust 'tube or towards the post-combustion chamber 3 according to whether the valve'16 is open or shut, that is to say according to whether the power which is available on the drive shaft of .the engine is high or low. The power limit above which the post-combustion can cease is comprised advantageously between a quarter and two-thirds of the maximal power of the engine.

'Referring to FIG. 2, one sees that the walls of the chamber of the post-combustion are constituted by those of a cylinder 18 of which the shape is such that the relationship of the length of the cylinder to its diameter is relatively small.

An end of the cylinder 18 is shut by a conical cap 19 whilst theother end is shut by a cover 20 in the form of a circular disc, appreciably flat and perpendicular to the axis of the cylinder 18.

The cover 20 comprises in its center a circular opening 21 in which is housed the end of the pipe 8 or a connection to this pipe. This latter, it is recalled links the exhaust of the engine to the chamber 3. It is as short a possible and is, preferably, blanketed, which allows maximum reduction of the heat losses of the exhaust gases between the engine and the post-combustion chamber 3.

The pipe 8' is prolonged coaxially inside the chamber 3, by a divergent frustoconic element 22, which extends appreciably over three-quarters of the length of the chamber and issues towards the cap 19. The diameter of the divergent member 22 increases from the pipe 8 towards the interior of the chamber 3.

Openings 23 are formed in the wall of the cylinder 18 in the region of the cover 20. I

The intermediate casing 11 is constituted by a cylinder 11a, coaxial to the cylinder 18 and ends, on the side of the cap 19, by a frustoconic wall 11b of which the end of small diameter is connected to the exhaust pipe 10.

A lug 24 ensures a liaison between the cap 19 and the cylinder 11a. 1

The end of the casing 11 opposite the wall 1 1b is shut by the cover 20.

An annular space 25, or intermediate zone, is thus defined between the cylinders 18 and 11a, which space communicates with the chamber 3 by the openings 23 and with the exhaust-tube'by the intermediary of the piping 10. v

This annular space 25 is traversed through, in the direction of the arrow F, by the exhaust gases of the post-combustion chamber.

The casing 4 is constituted by a cylinder coaxial to the casing 11 and is separated from it radially, towards the exterior, by a very small distance. The end of the casing 4 situated in the region of the piping ends by a tronconic wall 4b which rejoins the wall 11b. The other hand of the casing 4 is shut by a cover 26 slightly separated, towards the exterior, from the cover 20, the cover 26 being appreciably flat and at right angles to the axis of the cylinder 18.

A peripheral zone 27 of small thickness is limited by the casings 11 and 4, by the frustoconic walls 4b and 11b and by the covers and 26.

The orifices 9 of the piping 8 are spread out regularly on a circumference and issue into the zone 27, between the covers 20 and 26. The distance d following the-axis of the cylinder 18 between these covers is just sufficient for the orifices or the holes 9 to be able to issue between the said covers.

Preferably, the diameter of the pipe 8 at the level of the orifices 9 is minimal in such a way that the exhaust gases of the engine which direct themselves towards the chamber 3 form a convergent-divergent stream and produce an effect of vacuum at the level of the neck, that is to say at the level of the orifices 9.

It should be noted that the exhaust gases from the engine are introduced at the center and in the axis of the cylinder 18.

The piping 5, leading the air flowing from the blower,

crosses the wall 4b and issues into the peripheral zone 27, in the region of the connection of the walls 4b and 1 lb.

The conduit 17 issues into the peripheral zone 27 in the region of the cover 26. It will be noticed that the annular space separates the peripheral zone 27 from the combustion chamber 3 along the whole of the axial length of the latter.

The partition 12, disposed between the casings 11 and 4 is advantageously constituted by thread of cylindrical section 28 of which the diameter is appreciably equal to the distance separating the cylindrical walls of the casings 11 and 4. This thread is rolled into a helix over an axial distance appreciably equal to the distance separating the cover 20 from the base of the cover 19.

The assembly of the post-combustion chamber and of the casings 11 and 4 which constitute the heatexchanger 6 is made of metal and the different elements are preferably assembled by soldering.

The valve 16 able to open the conduit 17 towards the silencer when the power developed by the engine is sufficient, is shown schematically in mixed line.

The operation of the post-combustion chamber and of the heat-exchanger of FIG. 2 is as follows.

The exhaust gases from the engine arrive by the pipe 8 and the divergent element 22 into the chamber 3, as indicated by an arrow B.

If the power of the engine is less than the predetermined limit, the valve 16 is shut and the air flowing from the blower 2, which arrives by the pipe 5, runs through the zone 27 and crosses the holes 9 in order to mix itself with the exhaust gases of the engine of which the undesirable constituents burn, completely or partly, in the chamber 3.

The gases escape from the chamber 3 by the openings 23 and circulate in the space 25 according to the direction of the arrow F heating, in the process,

through the casing 11, the air flowing from the blower.

It will be noticed that, in the zone 27, the air moves longitudinally in counter-current to the exhaust gases of the post-combustion chamber.

If the power of the engine is equal to or exceeds the above-mentioned limit which is predetermined, the valve 16 opens and the air which flows through and from the blower, after having cooled the casing 11 and taken away the heat from the exhaust gases of the postcombustion chamber, escapes towards the silencer by the piping 17.

The quantity of air which flows from the blower is in excess in relation to that which is just sufficient to burn theoretically the undesirable constituents, in such a way that there subsists, after post-combustion, an excess of free oxygen of the order of 3 percent by volume.

The assembly is arranged in such a way that the heatexchangers between the exhaust gases and the additional air are sufficient to take the additional air when it arrives at the holes 9, to a temperature close to that of the post-combustion gases, which is of the order of 900 C, and in any case higher than 600 C.

The combustion reaction of carbon monoxide:

co+ a 0 co is a reaction of equilibrium which is moved in the direction of an augmentation of CO when the oxygen is in excess, which is thus favorable to the diminution of the amount of carbon monoxide.

It will be noted that there exists a compromise to be found for the flow of additional air, for a flow which is not sufficient does not permit the obtaining of excess of oxygen favorable to the combustion of carbon monoxide and of unburnt elements, and a flow which is too strong can lead to a drop of temperature which is finally detrimental. For these reasons, the content of oxygen of the exhaust gases of the post-combustion chamber must be somewhere between 1 percent and 8 percent, and preferably close to 3 percent, by volume.

When the power of the engine exceeds the fixed limit, the cooling of the combustion chamber and of the exhaust gases by the additional air is particularly advantageous because it allows a better holding-up of the materials and the welds, by diminishing the thermal stresses.

As regards the valve 16 (FIGS. 3, 4), it comprises advantageously a body 33, a sliding valve stem 34, of which the movement can be controlled mechanically by the aid of a lever 35 connected, on one hand, to an end of the stem 34 and, on the other hand, by liaison means such as link-rods, to the throttle valve of the gases or to the accelerator control.

The other end of the stem 34 carries a valve 36 (FIG. 3) or 36a (FIG. 4) which can isolate or make to communicate, according to whether it is shut or open, two chambers 37 and 38 provided in the body 33 and which are connected respectively to the conduit 17 and to a pipe 39 connected to the silencer.

The stem 34 slides in a guide 40 (FIG. 3) or 40a (FIG. 4) carried by the body of the valve 33.

Elastic return means 41, represented solely in FIG. 3, are provided to maintain shut the valve 36, outside of any external action. When the valve is maintained in its rest position by the said means, as represented on FIGS. 3 and 4, the chambers 37 and 38 are isolated from one another.

M16009 OOBI According to a first embodiment (FIG. 3), the valve 36 can co-operate with two seats 42, 43. The seat 42 is provided on the body of the valve 33 and can serve as a support to a complementary surface arranged on the edges of the face of the valve 36 opposite the stem 34.

The seat 43 is provided at the end of the guide 40 turned towards the valve 36 and can serve as support to a complementary surface 44 arranged on the edges of the surface of the valve 36 turned towards the stem 34.

In this way, at the time of the opening of the valve 36, the surface 44 comes into contact with the seat 43, which assures a good air-tightness between the valve and the guide 40 and prevents the hot air coming from the conduit 17 from escaping between the stem 34 and the guide 40.

According to a variation (FIG. 4), the guide 40a has a cylindrical elongated form and traverses a chamber 45 provided in the body 33.

This chamber 45 is transversed by the additional air before heating of the latter in such a manner that a cooling of the guide 40a of the stem 34 and of the body 33 is assured, which improves the holding-up of the valve 16'. It will be noticed that, in this variation, a sole seat 44a is provided on the body 33 for the valve 36a.

There is obtained finally a post-combustion device which is efficient, of simple construction and whose condition and resistance are very good since the throttle, constituted by the valve 16 allow, at high power, to stop the post-combustion by lack of additional air, which limits the temperature of the exit gases. Furthermore, again at high power levels, the air flowing from the blower cools, in the heat exhanger 6, the exhaust gases, which lowers their temperature of exit and assures a better holding-up of the device.

The post-combustion device is constructed in such a way that any cooling of the exhaust gases of the engine before their post-combustion is eliminated. This is due, in particular to the fact that'any exchange of heat between theexhaust gases of the engine and the additional air before they mix-is avoided.

The pre-heating of the additional air is due solely to the exhaust gases of the post-combustion chamberand the efficiency of the thermal exchanges is such that the additional air, inmixing with the exhaust gases of the engine, does not cool them in practice. There results a very good post-combustion which does not necessitate the presence of a spark plug in the post-combustion chamber 3.

Furthermore, advantage is taken of the fact that the additional air is well pre-heated in order to make the blower deliver an excess of air relative to the carbon monoxide to be burnt. This excess of air does not in practice cool the exhaust gases from the engine and allows the obtaining ofa complete combustion of the carbon monoxide and of the hydrocarbons.

The thermal inertia of the combustion device is minimal and allows a rapid putting into action of the latter.

' The exhaust gases of an internal combustion engine equipped with such a device have a content of carbon monoxide which passes from a value comprised between 0.5 and 4 percent by volume, before postcombustion, to a value somewhere between and 0.05 percent by volume, after post-combustion. The concentration of unburnt hydrocarbons in the exhaust gases which, measured according to the procedure of infrared absorption is comprised, before post-combustion, between and 4,000 ppm. (parts per million), is, after post-combustion, lower than 15 ppm.

As it goes without saying, and as' there results furthermore already from what has preceded, the invention is not limited in any way to that of its embodiments, neither to those embodiments of its diverse parts, which have been more particularly envisaged; it embraces, on the contrary, all variations.

I claim:

1. Post-combustion device for burning combustible gases contained in the exhaust gases of an internal combustion engine, comprising a post-combustion chamber devoid of a spark producing means and bounded by a casing,

means for introducing additional air into the postcombustion chamber,

a heat-exhanger adapted to pre-heat the additional air by exhaust gases from the post-combustion chamber, the heat-exchanger surrounding the said post-combustion chamber and being suchthat the circulation of additional air is effected in a peripheral zone in countercurrent to the exhaust gases which flow in an intermediate zone comprised between the said peripheral zone and the casing of the combustion chamber, the means for introducing the additional air comprising a blower adapted to be driven by the engine and to deliver a flow of additional air in excess, and the assembly of the post-combustion chamber and the heatexchanger being constituted by three coaxial cylinders namely an inner cylinder forming the casing of the post-combustion chamber and adapted to be connected to the exhaust pipe of the engine in such a way that the exhaust gases of the latter are introduced at the center and in the axis of said inner cylinder, an intermediate cylinder forming with the inner cylinder an annular space which constitutes said intermediate zone and communicates, at an end situated at the side of the engine exhaust pipe, with the post-combustion chamber by openings provided in the inner cylinder and, at itsother end, with a pipe for the escape of the burnt gases from the post-combustion chamber, an outer cylinder defining with the intermediate cylinder the abovesaid peripheral zone, which communicates, on one hand, with an inlet pipe for additional air and, on the other hand, with the exhaust pipe of the engine through holes provided in the latter and situated axially between two end-enclosing covers substantially flat and at right angles to the common axis of the cylinders, said covers being traversed by the exhaust pipe of the engine and being adapted to shut, at one end, on one hand the intermediate cylinder and the inner cylinder and, on the other hand, the outer cylinder, the axial distance between the covers being just sufficient for the holes to open between the said covers, said peripheral zone including a partition rolled in a helix, adapted to increase the path traversed by the additional air in said peripheral zone, the assembly being arranged in such a way that said intermediate zone separates the peripheral zone from the combustion chamber over the whole of the axial length of the latter, and

sensing means sensitive to at least one of (a) the speed and (b) the load of the engine comprising by-pass means located between the heat exchanger and the atmosphere to prevent the air delivered by the blower, which has cooled the exhaust gases from the post combustion chamber from being introduced into said post-combustion chamber when at least one of (a) and (b) has a value higher than a given limit to which corresponds a chosen power limit, in such a way that the post-combustion ceases when the engine delivers a power higher than said chosen power limit.

2. Post-combustion device according to claim 1, wherein the sensing means are connected to the position of the accelerator control of the engine.

3. Post-combustion device according to claim 1, wherein the sensing means are linked to the delivery pressure of the blower.

4. Post-combustion device according to claim 1, wherein the limit of power of the engine above which the sensing means are adapted to stop the post-combustion, is comprised between a quarter and two-thirds of the maximal power of the engine.

5. Post-combustion device according to claim 1, comprising a divergent frustoconical element coaxial with the post-combustion chamber and constituting an extension in the latter chamber, of the engine exhaust pipe, the diameter of the divergent element increasing from the pipe towards the interior of the post-combustion chamber.

this exchanger is higher than 600 C.

7. Post-combustion device according to claim 1, wherein the partition of the peripheral zone is constituted by a thread of cylindrical section.

8. Post-combustion device according to claim 1, wherein the flow of additional air delivered by the blower is such that the amount of oxygen in the exhaust gases at the exit from the post-combustion chamber is comprised between 1 percent and 8 percent by volume.

9. Post-combustion device according to claim 2, wherein the sensing means comprise a sliding valve provided with a valve flap carried by a stem and adapted to cooperate, on one hand, with a first seat when the valve flap is shut and, on the other hand, with a second seat carried by a guide of the stem of the said valve flap when the valve is open, the cooperation of the valve flap and of the seat being adapted to reduce the escapes of additional preheated air between the stem and the guide.

10. Post-combustion device according to claim 1, wherein the sensing means comprise a sliding valve provided with a valve flap carried by a stem, which stem is adapted to slide in a guide which crosses a cavity in which the additional cold air, circulated by the blower, is adapted to circulate in such a way that the guide and the stem are cooled. 

1. Post-combustion device for burning combustible gases contained in the exhaust gases of an internal combustion engine, comprising a post-combustion chamber devoid of a spark producing means and bounded by a casing, means for introducing additional air into the post-combustion chamber, a heat-exhanger adapted to pre-heat the additional air by exhaust gases from the post-combustion chamber, the heatexchanger surrounding the said post-combustion chamber and being such that the circulation of additional air is effected in a peripheral zone in countercurrent to the exhaust gases which flow in an intermediate zone comprised between the said peripheral zone and the casing of the combustion chamber, the means for introducing the additional air comprising a blower adapted to be driven by the engine and to deliver a flow of additional air in excess, and the assembly of the postcombustion chamber and the heat-exchanger being constituted by three coaxial cylinders namely an inner cylinder forming the casing of the post-combustion chamber and adapted to be connected to the exhaust pipe of the engine in such a way that the exhaust gases of the latter are introduced at the center and in the axis of said inner cylinder, an intermediate cylinder forming with the inner cylinder an annular space which constitutes said intermediate zone and communicates, at an end situated at the side of the engine exhaust pipe, with the postcombustion chamber by openings provided in the inner cylinder and, at its other end, with a pipe for the escape of the burnt gases from the post-combustion chamber, an outer cylinder defining with the intermediate cylinder the abovesaid peripheral zone, which communicates, on one hand, with an inlet pipe for additional air and, on the other hand, with the exhaust pipe of the engine through holes provided in the latter and situated axially between two end-enclosing covers substantially flat and at right angles to the common axis of the cylinders, said covers being traversed by the exhaust pipe of the engine and being adapted to shut, at one end, on one hand the intermediate cylinder and the inner cylinder and, on the other hand, the outer cylinder, the axial distance between the covers being just sufficient for the holes to open between the said covers, said peripheral zone including a partition rolled in a helix, adapted to increase the path traversed by the additional air in said peripheral zone, the assembly being arranged in such a way that said intermediate zone separates the peripheral zone from the combustion chamber over the whole of the axial length of the latter, and sensing means sensitive to at least one of (a) the speed and (b) the load of the engine comprising by-pass means located between the heat exchanger and the atmosphere to prevent the air delivered by the blower, which has cooled the exhaust gases from the post combustion chamber from being introduced into said post-combustion chamber when at least one of (a) and (b) has a value higher than a given limit to which corresponds a chosen power limit, in such a way that the post-combustion ceases when the engine delivers a power higher than said chosen power limit.
 2. Post-combustion device according to claim 1, wherein the sensing means are connected to the position of the accelerator control of the engine.
 3. Post-combustion device according to claim 1, wherein the sensing means are linked to the delivery pressure of the blower.
 4. Post-combustion device according to claim 1, wherein the limit of power of the engine above which the sensing means are adapted to stop the post-combustion, is comprised between a quarter and two-thirds of the maximal power of the engine.
 5. Post-combustion device according to claim 1, comprising a divergent frustoconical element coaxial with the post-combustion chamber and constituting an extension in the latter chamber, of the engine exhaust pipe, the diameter of the divergent element increasing from the pipe towards the interior of the post-combustion chamber.
 6. Post-combustion device according to claim 1, wherein the heat-exchanger is arranged in such a way that the temperature of the additional air at the exit of this exchanger is higher than 600* C.
 7. Post-combustion device according to claim 1, wherein the partition of the peripheral zone is constituted by a thread of cylindrical section.
 8. Post-combustion device according to claim 1, wherein the flow of additional air delivered by the blower is such that the amount of oxygen in the exhaust gases at the exit from the post-combustion chamber is comprised between 1 percent and 8 percent by volume.
 9. Post-combustion device according to claim 2, wherein the sensing means comprise a sliding valve provided with a valve flap carried by a stem and adapted to cooperate, on one hand, with a first seat when the valve flap is shut and, on the other hand, with a second seat carried by a guide of the stem of the said valve flap when the valve is open, the cooperation of the valve flap and of the seat being adapted to reduce the escapes of additional preheated air between the stem and the guide.
 10. Post-combustion device according to claim 1, wherein the sensing means comprise a sliding valve provided with a valve flap carried by a stem, which stem is adapted to slide in a guide which crosses a cavity in which the additional cold air, circulated by the blower, is adapted to circulate in such a way that the guide and the stem are cooled. 